Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 187

Glue should be of a bright brown or amber colour, free from specks or blotches, which are often caused by the non-removal of lime used in dressing the skins. It should be nearly transparent and with but little taste or smell. Black, opaque, unclean-looking glue is of no use. Very light-coloured glue is often fairly good and of medium price, but the bleaching to which it is subjected sometimes lowers its strength. For some purposes, such as gluing down thin, light-coloured veneers, it is very good, simply because it does not darken the tone of the wood, as the darker glue may do. Glue should be hard and moderately brittle, should not be readily affected by atmospheric damp, and should break sharply, with a glassy, shining appearance. If the glue shivers as easily as a piece of glass it is too brittle to be perfect, but at the same time it should not be tough and leathery. The appearance of the fractured edges also is often a good indication, as is also the feel when it is held or rubbed between a moistened finger and thumb. Good glue will not give off an unpleasant smell after being prepared a few days; some of the commoner kinds are very bad in this respect, the odour from them being unbearable. Good glue will not dissolve in cold water, but will swell and assume the consistency of jelly. For this reason some recommend as a test that the glue should be weighed, soaked, and washed in cold water, then dried and weighed again. If there is a loss of weight it shows that some has been dissolved, and according to the difference the quality may be judged. If the water dissolves it as it soaks in, and penetrates for a slight depth only, there is something wrong. Roughly speaking, a glue which will absorb more water than another is the one that is preferable. Glue is sometimes tested by gluing two pieces of wood end to end. Two pieces of mahogany exhibited at Bethnal Green Museum were tried in this way, and parted under a strain of 504 lb. per square inch. If two pieces of dry red deal be properly glued together side to side, or end to end, the wood will break before the joint if the glue be of fair average quality. The adhesive power of glue is in proportion to its consistency and elasticity after it has been soaked in water for some hours and has absorbed many times its own weight of moisture. For the woodworker’s purpose, Scotch glue is the best. In colour it is a clear, wholesome, ruddy brown, not a muddy-looking compound, nor yet refined to gelatine.

Fig. 481.—Section through Glue-pot.